Japan Nuclear Agency Secretly Calculated Fukushima Meltdown Risks

TOKYO – The Japanese nuclear safety agency secretly calculated the possibility of a worst-case meltdown at Tokyo Electric Power Co.’s Fukushima No. 1 power plant, it has been learned.

The agency was working on the calculations just as Tepco was saying the nuclear fuel in three reactors at the plant was “slightly damaged.”

The trial calculations were made under the premise that the nuclear fuel at the plant’s No. 1 to No. 3 reactors would melt down entirely, developing into a so-called “China syndrome,” the worst-possible scenario.

Coined in the United States, the term China syndrome refers to an imagined worst-case meltdown of nuclear fuel that burns through the bottom of a containment vessel and eventually through the Earth’s crust until finally reaching China.

The trial calculations in Japan were carried out first on March 25, two weeks after the March 11 accident resulting from an earthquake and tsunami, followed by further calculations on April 6, 7 and 13.

The fact that the calculations were carried out secretly was revealed Friday by the Japan Nuclear Energy Safety Organization (JNES), an independent administrative institution, which had been commissioned by the national Nuclear and Industrial Safety Agency to undertake the trial estimations.

The calculations indicated that if cooling water could not be injected, erosion could continue for more than 10 days, badly damaging the nearly 10-foot thick concrete walls of the Nos. 2 and 3 reactors’ containment vessels, the JNES said.

The erosion of the bottom of the No. 1 reactor’s containment vessel bottom would possibly stop after the vessel wall was eroded to a depth of 1.8 meters (almost 6 feet) in eight days, according to the calculations.

When the JNES calculations were being conducted, both the agency – the nuclear safety watchdog of the Economy, Trade and Industry Ministry – and Tepco publicly said the nuclear fuel in the reactors was “slightly damaged.”

In announcing the calculation results Friday, the JNES said it had roughly gauged the extent to which the floor of the containment vessel bottoms, called “pedestals,” would be eroded should all the nuclear fuel drop through the pressure vessel of the three reactors.

The calculations were carried out based on different scenarios. In one of them, all reactor cores dropped out at the same time, and in another, melted fuel jetted out of the vessels, the JNES said.

It said the calculation results were conveyed to the agency, but it is unknown whether the information was shared with the Cabinet Office or other government organizations.

Regarding the announcement of the calculations a half year after the outbreak of the crisis, the JNES said that although the calculations were exclusively for the sake of studies within the agency and Tepco, it decided to make them public as the initial phase of coping with the crisis has finished.

On April 18 the agency acknowledged the reactors’ fuel had begun to melt down, but Tepco failed to acknowledge the possibility of a meltdown until April 20.